An electromechanical power transmission chain comprises typically one or more electrical machines and an electronic power converter. The electromechanical power transmission chain can be a series transmission chain where one of the electrical machines operates as generator and the electronic power converter is arranged to convert the electrical voltages produced by the generator into electrical voltages having amplitudes and frequencies suitable for the one or more other electrical machines. The generator can be driven with a combustion engine that can be e.g. a diesel engine, an Otto-cycle engine, or a turbine engine. The other electrical machines can be, for example, electrical motors in wheels of a mobile working machine. The electronic power converter comprises typically an intermediate circuit, an electronic power converter stage between the generator and the intermediate circuit and one or more other electronic power converter stages between the intermediate circuit and the other electrical machines. It is also possible that the electromechanical power transmission chain is a parallel transmission chain where an electrical machine that is mechanically connected to a combustion engine operates sometimes as a generator which charges one or more energy-storages and sometimes as a motor that receives electrical energy from the one or more energy-storages and assists the combustion engine when high mechanical output power is needed. In this case, the electronic power converter comprises typically an intermediate circuit, an electronic power converter stage between the generator and the intermediate circuit, and one or more electronic power converter stages between the intermediate circuit and the one or more energy-storages.
The above-mentioned intermediate circuit is typically a capacitive circuit capable of storing electrical energy. In order that the electronic power converter stages of the electronic power converter would be able to operate properly, the voltage of the intermediate circuit needs to be between appropriate lower and upper limits which can be, for example, 300 V and 750 V, respectively. The electrical energy stored by the intermediate circuit is directly proportional to the square of the voltage of the intermediate circuit. In many cases, the reference level of the voltage of the intermediate circuit is advantageously chosen so that electrical energy corresponding to the reference level of the voltage is in the middle between electrical energy corresponding to the lower limit of the voltage and electrical energy corresponding to the upper limit of the voltage. Due to the above-mentioned square-dependency, the reference level of the voltage is not in the middle between the upper and lower limits of the voltage. For example, in the above-mentioned exemplifying case, where the lower and upper limits are 300 V and 750 V, respectively, the reference level of the voltage is about 571 V. Thus, there is a 271 V safety margin between the reference level and the lower limit but only a 179 V safety margin between the reference level and the upper limit. An inconveniency related to the asymmetry of these safety margins is that it complicates the control of the voltage of the intermediate circuit. A similar inconveniency is present also in conjunction with inductive intermediate circuits because the electrical energy stored by an inductive intermediate circuit is directly proportional to the square of the current of the intermediate circuit. It is naturally possible to use the square of the capacitor voltage or the square of the inductor current as a control quantity but, in this case, the square-type non-linearity is included in the control quantity, which in turn may complicate the control.